Physical camera models are used to represent the geometric relationship between the positions of objects in an image (image space coordinates) and the corresponding positions of the objects in real space (object space coordinates). To create a physical model for a camera, details such as the focal length, principal point location, lens distortion, and other factors must be known about the camera. In some cases, this information is proprietary information that is not readily available to the public. Additionally, these models may be highly complex mathematical models that require a great deal of effort to implement in software packages, such as software packages used to perform photogrammetric processing (e.g., software to generate Digital Elevation Models or topographical maps).
An alternate sensor model that may be used instead of a physical camera model is the Rational Polynomial Camera (RPC) Model. The RPC Model is a generic sensor model that can be used with a wide range of sensors. Additionally, it is widely supported in commercial software packages. However, the use of the RPC model requires values for a set of coefficients used in the model. These coefficients are generated using the physical camera model. Thus, to use the RPC model, either 1) the physical camera model (or knowledge of the interior configuration of the sensor) must be known, or 2) the coefficients used in the model must be provided by the camera manufacturer.